The invention relates to a DCxe2x80x94DC converter for generating a higher, second DC voltage from a first DC voltage, which converter comprises
a first input terminal which is to be connected to the positive pole of a voltage source supplying the first DC voltage,
a second input terminal which is to be connected to the negative pole of the voltage source supplying the first DC voltage,
a first branch which interconnects the input terminals and comprises a series arrangement of an inductive element L and a switching element S1,
a control circuit, which is coupled to a control electrode of the switching element S1, and which is intended to generate a control signal for rendering the switching element S1 conducting and non-conducting,
a second branch which connects a point of the first branch between the inductive element L and the switching element S1 to the second input terminal, and which comprises a series arrangement of a unidirectional element D1 and an output capacitor C1.
The invention also relates to a circuit arrangement for feeding a discharge lamp.
A DCxe2x80x94DC converter as mentioned in the opening paragraph is commonly referred to as up-converter or boost-converter and is frequently applied in, for example, electronic ballast circuits for feeding a discharge lamp. The known DCxe2x80x94DC converter is frequently operated in the so-termed xe2x80x9ccritical modexe2x80x9d. This means that, in each period of the control signal, the switching element is rendered conducting during a constant time interval. Subsequently, the switching element is rendered non-conducting, after which it is rendered conducting again as soon as the current through the inductive element has become equal to zero. Operating said converter in the xe2x80x9ccritical modexe2x80x9d has the advantage that the power dissipation in the unidirectional element is comparatively small while, at the same time, the frequency of the control signal is comparatively high, so that the inductive element L can be chosen to be comparatively compact.
The known DCxe2x80x94DC converter is frequently applied in, for example, electronic ballast circuits for feeding a discharge lamp. In this application, and in many other applications, the first DC voltage is generally formed by a full-wave rectified low-frequency AC voltage supplied by the mains. To satisfy statutory requirements regarding power factor and THD, it is necessary for the current drawn from the mains by the DCxe2x80x94DC converter to be an alternating current having the same frequency and approximately the same shape as the low-frequency AC voltage. In addition, this current must be in phase with the low-frequency AC voltage. In the case of the known DCxe2x80x94DC converter, which is operated in the xe2x80x9ccritical modexe2x80x9d, these two requirements cause the frequency with which the control signal renders the switching element conducting and non-conducting to depend very substantially on the instantaneous amplitude of the low-frequency AC voltage and the power drawn from the output capacitor. In practice it has been found that the integrated circuits that are generally applied in the control circuits of the known DCxe2x80x94DC converter set a practical limit to the frequency of the control signal. At frequencies above this limit, instabilities occur in the operation of the DCxe2x80x94DC converter that may lead to damage. As a result of this limit the DCxe2x80x94DC converter must be designed such that, even in the vicinity of the zero crossings of the low frequency AC voltage and at a small value of the power supplied by the DCxe2x80x94DC converter, the frequency of the control signal does not go beyond this limit. As a result, the frequency of the control signal is comparatively low at a comparatively high instantaneous amplitude of the low-frequency AC voltage and a comparatively high value of the power supplied by the DCxe2x80x94DC converter. This means that, for example, the inductive element L must be chosen to be comparatively large, which causes the DCxe2x80x94DC converter to become voluminous and expensive. Another drawback of the known DCxe2x80x94DC converter resides in that xe2x80x9chard switchingxe2x80x9d occurs if the instantaneous amplitude of the first DC voltage is higher than half the voltage present across the output capacitor. Hard switching means that the switching element is rendered conducting by the control signal while a comparatively high voltage is present across the switching element. This leads to a comparatively high power dissipation in the switching element, which may also cause damage to the switching element.
U.S. Pat. No. 6,194,880 (Fraidlin et al.) discloses a DCxe2x80x94DC converter in which current flows bi-directionally through a boost inductor.
It is an object of the invention to provide a DCxe2x80x94DC converter wherein hard switching is effectively counteracted, and which can be embodied so as to be comparatively compact and inexpensive.
To achieve this, a DCxe2x80x94DC converter as mentioned in the opening paragraph is characterized in accordance with the invention in that the DCxe2x80x94DC converter is further provided with
a third branch which comprises a further unidirectional element D2, and which connects the second input terminal to a point of the first branch between the inductive element L and the switching element S1,
a fourth branch which shunts the unidirectional element D1, and which comprises a further switching element S2 a control electrode of which is coupled to the control circuit,
and in that the control circuit is provided with means for alternately rendering the switching element S1 and the further switching element S2 conducting and non-conducting with a frequency f, thereby causing the current through the inductive element L to change direction twice in each period of the control signal.
By virtue of the presence of the further switching element, it is possible to design a DCxe2x80x94DC converter in accordance with the invention in such a manner that hard switching is precluded at any value of the power supplied by the DCxe2x80x94DC converter and at any ratio between the instantaneous amplitude of the first DC voltage and the voltage across the output capacitor. It has also been found that if the DCxe2x80x94DC converter is fed from a supply voltage source supplying a low-frequency AC voltage, the frequency of the control signal does not have to be changed over a large range of both the power supplied by the DCxe2x80x94DC converter and the instantaneous amplitude of the first DC voltage to meet high requirements regarding power factor and THD. By virtue thereof, the frequency of the control signal can be chosen to be at a constant high value so that, for example, the inductive element L can be chosen to be small and the DCxe2x80x94DC converter can be embodied so as to be compact and inexpensive.
Said advantages are used in a preferred embodiment of a DCxe2x80x94DC converter in accordance with the invention, wherein the DCxe2x80x94DC converter is additionally provided with
a fifth branch which comprises an input capacitance and interconnects the first and the second input terminal,
a rectifier respective output terminals of which are connected to the first and the second input terminal, and which is provided with rectifier input terminals which are to be connected to a supply voltage source supplying a low-frequency AC voltage,
and the control circuit is provided with means for controlling each one of the switching elements in each period of the control signal in such a manner that the current drawn from the supply voltage source is an alternating current of the same frequency as the low-frequency AC voltage, and is in phase with the low-frequency AC voltage.
This preferred embodiment can very suitably be used in circuit arrangements that are fed from the mains that supply a low-frequency sine-shaped AC voltage.
In a further embodiment of a DCxe2x80x94DC converter in accordance with the invention, the control circuit is provided with
a microprocessor for generating, in dependence on the instantaneous amplitude of the low-frequency AC voltage and the power drawn from the output capacitor, a first signal that is a measure of the maximum amplitude Ipos of the current through the inductive element L in a first direction, and a second signal that is a measure of the maximum amplitude Ineg of the current through the inductive element L in a second direction,
a signal generator for generating a signal that is a measure of the instantaneous amplitude of the current through the inductive element L, and
a driver, coupled to the signal generator and the microprocessor, for rendering the switching element S1 conducting in a time interval wherein the further unidirectional element D2 carries current, and for rendering the switching element S1 non-conducting when the amplitude of the current through the inductive element L is equal to Ipos, and for rendering the further switching element S2 conducting in a time interval wherein the unidirectional element D1 carries current, and for rendering the second switching element non-conducting when the amplitude of the current through the inductive element L is equal to Ineg. The value of Ipos and of Ineg depend substantially on the power supplied by the further preferred embodiment and on the instantaneous value of the first DC voltage. Ipos and Ineg also depend on the frequency f of the control signal and the voltage across the output capacitor. However, the latter are frequently constant. In the further preferred embodiment, the control circuit is embodied, in a comparatively simple and reliable manner, such that the further preferred embodiment has a high power factor and causes only a small quantity of THD.
As a result of the advantageous properties indicated hereinabove, a DCxe2x80x94DC converter in accordance with the invention can very suitably be used in a circuit arrangement for feeding a lamp. This applies, more particularly, when the circuit arrangement is additionally provided with a circuit part for adjusting the power consumed by the lamp. If the power consumed by the lamp can be adjusted over a large range, then the DCxe2x80x94DC converter must be capable of functioning well over a large range of the power supplied. As indicated hereinabove, a DCxe2x80x94DC converter in accordance with the invention is capable of meeting high requirements regarding power factor and THD over a large range of the power supplied and at a constant frequency.